Falls in the elderly are common and are hazardous. Almost one-third of community-dwelling persons 75 or more years old fall each year, and more than five percent of those who fall experience fractures. Tendencies of old adults to fall are associated with increases in their postural sway. Both fall and postural sway responses, regardless of their underlying neurologic or musculoskeletal pathologies, can be quantified in biomechanical terms. Continuation is proposed of research to examine the response biomechanics of the musculoskeletal system in the arrest of impending falls and the maintenance of postural balance. Small, moderate and large disturbances of standing posture will be made. Sway responses and stepping responses will result. Body segment kinematics, myoelectric activities and support surface reactions will be measured. Angular momenta, kinetic energies and joint torques will be computed through biomechanical model analyses. The overall objectives are to quantify differences in response biomechanics among healthy young, old and old-old adults, and old adults with disease-related mobility impairments; to determine to what extent those differences arise from natural aging as opposed to disease processes, and to seek specific biomechanical sources of those differences. An understanding of why old adults fall more often than young will aid programs for prevention of mobility impairments, improve assessments of patient risk, and facilitate the design of effective therapies.